In the past, the magneto-optical memory medium has been applied as a rewriteable optical memory medium. In this type of magneto-optical memory medium, information recorded in the magneto-optical memory medium is reproduced by focusing thereon a light beam projected by a semiconductor laser. A problem with this type of magneto-optical memory medium is that reproducing characteristics deteriorate if the diameter and interval between recording bits, which are magnetic domains for recording, become too small with respect to the diameter of the light beam.
This problem is caused by inability to distinguish and reproduce individual recording bits, because adjacent recording bits fall within the diameter of the light beam focused on a target recording bit.
In order to solve the foregoing problem, Japanese Unexamined Patent Publication No. 6-150418/1994 (Tokukaihei 6-150418, corresponding to U.S. patent application Ser. No. 08/147,373) (hereinafter referred to as "Conventional Example 1") proposes a magneto-optical memory medium including a non-magnetic intermediate layer provided between a reproducing layer, which has in-plane magnetization at room temperature but changes to perpendicular magnetization with a rise in temperature, and a recording layer, in which the reproducing layer and the recording layer are magnetostatically coupled.
By this means, since areas of the reproducing layer which are magnetized in plane mask information recorded in magnetic domains of the recording layer, individual recording bits can be distinguished and reproduced even if adjacent recording bits fall within the diameter of the focused light beam.
Again, "Magnetic domain expansion readout for amplification of an ultra high density magneto-optical recording signal," Appl. Phys. Lett. 69(27), pp. 4257-59 (hereinafter referred to as "Conventional Example 2"), discloses a structure in which a non-magnetic intermediate layer is provided between a reproducing layer and a recording layer. In this structure, by means of a magnetic field produced by the recording layer, the information of the recording layer is copied to a magnetic domain in the reproducing layer larger than the magnetic domain of the recording layer. This conventional art discloses a magnetic domain expansion method, in which, by copying the information of the recording layer to a larger magnetic domain in the reproducing layer, reproducing is carried out through the reproducing layer.
However, with Conventional Example 1, it has been confirmed that, when performing recording and reproducing using even smaller recording bits at closer intervals, the strength of the reproducing signal decreases, and a sufficient reproducing signal cannot be obtained.
Again, a problem with Comparative Example 2 is that, with high recording density, when many recording bits lie beneath a magnetic domain of the reproducing layer, the magnetic fields of multiple recording bits are applied to the reproducing layer, making it impossible for the magnetization of a single target recording bit to be correctly applied to the reproducing layer, leading to deterioration of reproducing signal quality.
Japanese Unexamined Patent Publication No. 1-143041/1989 (Tokukaihei 1-143041) discloses a magneto-optical memory medium in which information written in a recording layer is reproduced by copying it to a larger magnetic domain in a reproducing layer. In this medium, by means of magnetic domain expansion reproducing, the reproducing signal can be strengthened. In addition, Japanese Unexamined Patent Publication No. 8-7350/1996 (Tokukaihei 8-7350) and Abstracts of Papers of Twentieth Japan Applied Magnetics Conference (1996), 22pE-4, p. 313 propose a magneto-optical memory medium in which a pulse magnetic field synchronized with the recorded signal is applied during reproducing, to expand the magnetic domain during reproducing and reduce it thereafter, thus increasing the strength of the reproducing signal.
However, with the foregoing conventional art disclosed in Japanese Unexamined Patent Publication No. 1-143041/1989, Japanese Unexamined Patent Publication No. 8-7350/1996, and Abstracts of Papers of Twentieth Japan Applied Magnetics Conference (1996), 22pE-4, p. 313, when the interval between recording bits is small, adjacent bits fall within the diameter of the light beam focused on the target bit, the operations of magnetic domain expansion reproducing become unstable, as in the cases of Conventional Examples 1 and 2, and sufficient reproducing signal quality cannot be obtained.